1. Field of the Invention
The present invention relates to an injection molding machine used in the case of melting low-melting point nonferrous metals such as zinc, magnesium, or an alloy consisting thereof completely to perform injection molding under the condition of a liquid phase.
2. Detailed Description of the Prior Art
Die casting has been used for a minting of the low-melting point nonferrous metal, however, a melt furnace for melting a metallic material completely is required in die-casting, and it has been performed by dipping out a molten metal from this melt furnace or extruding by using a plunger. Accordingly, without using a melting furnace in the same manner as the case of plastic material, it has been performed to inject and fill into a mold from a nozzle on a tip of the heating cylinder by advancing of the screw, after melting in a heating cylinder which a screw for injection is provided rotatably and movably in the axial direction to melt the powdered metallic material applied from the rear of the heating cylinder completely while transferring toward the front of the heating cylinder by rotation of the screw and to store in an antechamber of the heating chamber under the condition of the liquid phase and to weigh.
Problems in the case of adopting such injection molding for the metallic material are caused by difficulty in melting and transferring of the metallic material by rotation of the screw and unstableness in weighing.
Since most of melting in the plastic material are caused by generation of heat by shear, the screw is formed in a large diameter as it comes to a tip portion and a screw groove providing a flowing clearance for the material is formed relatively shallowly. However, since there is a difference in a frictional factor in a boundary surface of an inner wall of the heating cylinder in a molten plastic, transferring toward the front by rotation of the screw can be performed smoothly, even though the flowing clearance is formed narrowly.
In contrast to this, since the metallic material melted up to the condition of the liquid phase completely is small in a viscosity to the extent not to be compared with the plastic material, the difference in the frictional factor at the two boundary surfaces described above is practically nothing, and a transferring force by rotation of the screw such as the case of the molten plastic is hard to cause due to this reason.
Moreover, in the plastic material, it becomes high viscosity due to melting, and since a pressure caused by material which pushes back the screw to the rear is occurred as a reaction force, as being stored in the antechamber of a melting cylinder by revolution of the screw, weighing of the molten material can be controlled into a constant amount each time by controlling this retracting of the screw due to pressure caused by material, however, since a rise in pressure up to such extent that the screw is pushed back to the rear is not caused in the liquid phase that the metallic material is in low viscosity, retracting of the screw due to the pressure caused by material is hard to occur, and an amount to be stored into the antechamber also is varied, whereby weighing can not be controlled into a constant amount each time.
Moreover, the heating cylinder is heated by a band heater of the outer peripheral to maintain a predetermined temperature, however, since there is no heating means in the screw side, and it is in the condition easy to radiate heat from a rear end which a piston rod is coupled, nonuniformity in temperature is easy to occur in the molten metal within the screw groove, and it leads to an excessive supply of material to keep the screw revolving in order to prevent this, since the screw itself is combined with a material-transferring member through the revolution, therefore, it has been impossible.